Semiconductor integrated circuits are fabricated in large numbers in successive layers of a semiconductor substrate. The integrated circuits are arranged in groups known as semiconductor dies, on the surface of the substrate. The substrate covers a silicon wafer.
One type of silicon wafer has a circular circumference that is truncated by a flat edge referred to as a wafer flat. The positions of the semiconductor dies are referenced to the wafer flat and the wafer center. Thus, the wafer flat is a reference, as is the wafer center.
During automated fabrication, a wafer holder or clamp grips the wafer by its edges. The wafer holder or clamp orients the wafer and guides the wafer through automated wafer fabrication equipment. The wafer flat is oriented relative to the clamp before the clamp transfers the wafer to the wafer fabrication equipment.
When the wafer is oriented improperly relative to the wafer fabrication equipment, the equipment can produce defects on the wafer surface, and some of the dies can become covered by the clamp and, thus, shielded from proper fabrication.
Prior to the invention, an edge detector detected the position of the wafer flat. However, the wafer might shift out of position, or the edge detector might fail to operate. There was no mechanism to detect the wafer shift, or to shut down the automated fabrication equipment upon detection of a wafer shift.